A fluid dynamic bearing device supports a shaft member in a freely rotating manner with a lubricating film of fluid formed in a bearing gap. The fluid dynamic bearing device has characteristics of high-speed rotation, high rotational accuracy, low noise, and the like, and is recently being suitably used as a bearing for a motor mounted on various electric equipment such as information equipment, specifically, for a spindle motor of a magnetic disk device including an HDD, an optical disk device including a CD-ROM, a CD-R/RW, and a DVD-ROM/RAM, and a magneto optical disk device including MD and MO, for a polygon scanner motor of a laser beam printer (LBP), for a color wheel motor of a projector, and for a fan motor.
Examples of the fluid dynamic bearing device that is incorporated in the spindle motor for the disk device include one having the structure illustrated in FIG. 14. In the fluid dynamic bearing device illustrated in FIG. 14, a region that constitutes a radial bearing surface is provided at each of the two upper and lower points on an inner peripheral surface 208a of a single bearing sleeve 208 fixed to a housing 207, the radial bearing surface being provided with a dynamic pressure generating part. In addition, radial bearing parts R and R are formed between the radial bearing surfaces and an outer peripheral surface 201a of a shaft member 200 (shaft part 201). Further, regions that constitute thrust bearing surfaces are provided on both end surfaces 208b and 208c of the bearing sleeve 208, the thrust bearing surface being provided with a dynamic pressure generating part. In addition, thrust bearing parts T and T are formed between the thrust bearing surfaces and an end surface 202a of a flange part 202 which is provided while protruding on the outer diameter side of the shaft part 201, and an end surface 209b of a seal member 209 (refer to Patent Document 1, for example).
Note that, regarding the inner periphery of the housing, other than the above-mentioned case where the bearing sleeve is fixed at one point in the axial direction, there may be given a case where the bearing sleeve is fixed at two points in the axial direction (refer to Patent Document 2, for example). Further, there may be given a case where the spacer (also referred to as distance piece) is fixed between both the bearing sleeves which are fixed at the two points axially separated from each other (refer to Patent Document 3, for example).    Patent Document 1: JP 2005-321089 A    Patent Document 2: JP 11-269475 A    Patent Document 3: JP 11-155254 A
The rotational accuracy of the fluid dynamic bearing device is largely influenced by accuracy of width of the radial bearing gap of the radial bearing part and the thrust bearing gap of the thrust bearing part. Therefore, it is necessary to assemble a bearing sleeve to a housing with accuracy. However, in the above-mentioned case where the bearing gaps are formed on the inner peripheral side and both the end sides of the bearing sleeve, it is necessary to fix the bearing sleeve to the housing while taking both the radial direction and the axial direction into consideration. Thus, the manufacturing cost thereof has sharply risen under present circumstances.
Further, in accordance with the increase in information processing amount and the like, the motor for information equipment including the above-mentioned spindle motor has been improved such that the lamination of the recording media and the high speed rotation can be achieved. In accordance therewith, it is necessary to impart to the fluid dynamic bearing device higher bearing strength and, in particular, more excellent load resistance (moment rigidity) to the moment load.
Incidentally, the structure in which the bearing sleeve is fixed at two points in the axial direction as described in Patent Documents 2 and 3 described above is advantageous for increasing moment rigidity when compared with that described in Patent Document 1. In contrast, it is necessary to give particular consideration to the accuracy in assembling between the bearing sleeves. This is because, when the bearing sleeve is fixed while being axially displaced, there may be risks that the arrangement spaces for the seal member and the lid member cannot be secured, and that a predetermined thrust bearing gap cannot be formed on the end side of the bearing sleeve. Further, this is also because, when both the bearing sleeves are fixed while being radially displaced from each other, the coaxiality between the radial bearing surfaces cannot be secured, which leads to the problems of the deterioration in rotational accuracy, the partial wear of the bearing sleeve, and the like. However, the housing and the bearing sleeve have dimensional tolerances, and hence it is difficult to perform positioning of those members with accuracy.